This raises an interesting question: Given that Bostonians (and New Englanders in general) were clearly Yankees as of 1865 (the end of the Civil War), and given that the New York baseball team was renamed the "Yankees" in 1913, at what point did the latter meaning start to predominate? It can't have been right away, but clearly by 2015 the baseball meaning is practically the only association people (at least in the Northeast) have with the term.